1. Field of the Invention
The present invention relates to a molten carbonate fuel cell and, more particularly, to a molten carbonate fuel cell in which an electrolyte body sandwiched between a pair of conductive electrodes is improved.
2. Description of the Related Art
A basic structure of a molten carbonate fuel cell is shown in FIG. 1. An electrolyte body 3 retaining an electrolyte consisting of an alkali carbonate is sandwiched between an anode (fuel electrode) 1 and a cathode (air electrode) 2 which serve as a pair of conductive electrodes. Two housings 4a and 4b abut against peripheral portions of both surfaces of the electrolyte body 3. The anode 1 and the cathode 2 are stored in the housings 4a and 4b, respectively. Corrugated collectors 5a and 5b are arranged between the inner surface of one housing 4a and the anode 1 and between the inner surface of the other housing 4b and the cathode 2, respectively. A supply port 6 for supplying a fuel gas (H.sub.2 and CO.sub.2) and an exhaust port 7 for exhausting an exhaust gas (CO.sub.2 and H.sub.2 O) are formed in the housing 4a in which the anode 1 is arranged. A supply port 8 for supplying an oxidant gas (air and CO.sub.2) and an exhaust port 9 for exhausting an exhaust gas (N.sub.2) are formed in the housing 4b in which the cathode 2 is arranged.
In the molten carbonate fuel cell shown in FIG. 1, an alkali carbonate in the electrolyte body 3 is melted at a high temperature. The fuel gas (H.sub.2 and CO.sub.2) is supplied to the anode 1 through the supply port 6 of the housing 4a, while the oxidant gas (air and CO.sub.2) is supplied to the cathode 2 through the supply port 8 of the housing 4b. Reactions represented by formulas (1) and (2) are performed at the anode 1 and the cathode 2, respectively: EQU H.sub.2 +CO.sub.3.sup.-2 .fwdarw.H.sub.2 O+CO.sub.2 +2e.sup.-( 1) EQU 1/2+CO.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.-2 ( 2)
An electrolyte body used in the above molten carbonate fuel cell conventionally comprises an electrolyte consisting of an alkali carbonate mixture and an electrolyte retaining material for preventing the electrolyte from flowing out from the electrolyte body since the electrolyte is melted during a high-temperature operation. The alkali carbonate mixture is used as a mixture of two or three salts selected from Li.sub.2 CO.sub.3, K.sub.2 CO.sub.3, and Na.sub.2 CO.sub.3. The electrolyte retaining material is a fine powder having a particle size of 0.05 to 0.5 .mu.m and consisting of .gamma.-LiAlO.sub.2 and .beta.-LiAlO.sub.2. The electrolyte body allows the passage of carbonic acid ions (CO.sub.3.sup.-2) and also serves as a gas permeation barrier layer for preventing direct mixing (gas crossover) of the reaction gases between the anode and the cathode. In order to provide these functions, it is necessary to sufficiently retain the electrolyte in the electrolyte body. An outflow of the electrolyte (electrolyte loss) increases an internal resistance and causes gas crossover.
In the electrolyte body having the above composition, however, an electrolyte loss progresses during the operation of several thousands of hours to shorten the life time of the cell. The electrolyte loss is caused by (1) evaporation of the electrolyte in a gaseous state and (2) outflow of the electrolyte in a liquid state. The cause (2) is considered to be a major cause of the electrolyte loss. The outflow of the electrolyte in the liquid state occurs because the electrolyte retaining material cannot sufficiently retain the alkali carbonate.
A conventional technique for improving an electrolyte retaining material of the electrolyte body or increasing a specific surface area is employed. However, the outflow of the electrolyte cannot be effectively prevented.
A technique for improving the electrolyte body is reported by C. Y. Yuh and A. Pigeaud, "Determination of Optimum Electrolyte Composition for Molten Carbonate Fuel Cells Literature Review" (1987), DOE/MC/23264-2507 (DE88001015). This literature describes that an electrolyte body added with 0.5 wt % of MgO and 0.5 wt % of BaO is used to suppress the dissobution of NiO from a cathode consisting of a nickel oxide. Another technique for improving an electrolyte body is reported by Joel D. Doyon, Thomas Gilbert, Geottrey Davis, and Lawrence Paetch, J. Electroche, Soc., 34 (1987) 3035-38. In this literature, an electrolyte body added with 1 wt % of MgO and SrO is used to suppress the dissolution of NiO from a cathode consisting of a nickel oxide. In addition, still another technique for improving an electrolyte body is proposed by Hideyuki Ohzu et al., "Effect of Group II-A Element Addition on Reduction of Elution of Cathode for Molten Carbonate Fuel Cell", 1987, Lecture Papers, Corp. Yogyo Kyokai. In this literature, an electrolyte body added with 5 mol % of MgCO.sub.3 is used to suppress the dissolution of Ni from a cathode consisting of a nickel porous sintered body.